During the summer, when the outside temperature is typically much higher than the inside temperature,
attic ventilation serves to help cool your house. An unfinished attic builds up a tremendous amount of heat, and if that heated
air has no place to escape, it can make the inside of the house much warmer or cause an air conditioning system to work much
harder to cool the house.

There are a wide variety of sources of moisture in a home, from the building materials themselves to normal everyday activities. For example
cooking, bathing and washing clothes all release gallons of water vapor into the air.

That vapor isn't a problem inside the average home because the temperature inside the home is warmer than outside for much of the
year. Warm air holds more moisture in the form of water vapor than cool air.

The problem is that vapor gradually works its way out of the living area and into the structure. As warm, moist air cools, the vapor begins to condense into water
droplets. If that happens inside an unfinished attic, for example, it can get insulation and framing materials wet. That not only reduces the value of your insulation but can cause mold, mildew
and rot.

Building codes specify the minimum amount of attic ventilation needed in a new home to prevent winter moisture buildup, but your summer needs are much greater. Also, older homes were often built
with inadequate attic ventilation at least by today's standards and may need to be retrofitted with proper attic ventilation.

A good attic ventilation system is designed for summer needs and moisture. It includes two types of vents: intake vents are placed
along the soffit to allow fresh air into the attic, and exhaust vents are installed in the upper third of the roof to allow attic
air to escape. The object is to create a continuous "wash" of air along the underside of the roof sheathing. The rule of thumb for
summer months is that you should provide enough ventilation to completely change the air in your attic every six minutes.

Gable Vents can provided in most any shape to compliment house architecture. Gable vents are installed in a gable wall just below the peak of the roof. These vents are cost effective yet provide a lot of net free vent area. They can
function as both intake and exhaust vents, depending on the roof design.

Rectangular Soffit Vents are cost effective and designed to fit the space between rafters and roof. They can provide a lot of intake air with 55 to 78 sq.in. of net free area. They perform best when used together with vent tubes
(to prevent attic insulation from blocking intake). Standard sizes are 22.25" wide x 3.5" or 5.0" high and designed to fit standard 24" rafters. They are sold 50 per bundle. These are easy to install. Simply cut rectangular holes in the soffit and screwing the vent over the hole.
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Circular Soffit vents range from 1" to 8" in diameter. They are installed by drilling holes in the soffit and pressing the vent into the hole. Generally, these do not provide anywhere near sufficient intake and suffer from blockage by attic insulation.

Static vents, also known as roof line or eyebrow vents, consist of a sheet metal cylinder with a flashing collar and a metal hood to keep rain out and an inside bug screen.. They are installed in rows along the face of the roof by cutting holes in the roof, nailing the flashing collars to the roof sheathing and shingling around the vents. Their effectiveness depends on how many are installed; probably their
greatest disadvantage is that like any roof
penetration, they may leak.

Exhaust vents fall into two basic categories. Static vents simply allow air to escape while power ventilators actively suck air out of the attic. Within each category there are a
number of types:

Gable vents can provided in most any shape to compliment house architecture. Gable vents are installed in a gable wall just below the peak of the roof. These vents are cost
effective yet provide a lot of net free vent area. They can function as both intake and exhaust vents, depending on the roof design.

Ridge vents are installed along the peak of the roof and replace the ridge singles. More advanced ridge vents, such as
VariVents, offer superior performance together with excellent rain resistance.

Wind Powered Ventilators are turbine vents or Aura vents. They
are installed like roof line vents along the face of the roof. When the wind blows, it spins the turbine, which in turn draws air up out of the
attic, or in the case of Aura vents, the wind creates negative pressure pulling the air from the attic. Their effectiveness, largely depends on whether the wind is blowing or not.

Fan-driven ventilators or Powered Vents are powered by electricity and usually controlled by a thermostat in the attic. They are very effective, but since they are motor-driven, the
extra cost of running them partially offsets the energy they conserve, and motors can have some maintenance issues.

The number of vents you'll need depends on the type, size and effectiveness of the vents. Vents are rated according to their square inches of
net "free vent area" (FVA) in other words, the net size of the flow through opening. You can't just measure the size of the vent to find the FVA because the open space is reduced by louvers and by the screen
mesh that covers the opening.

Most manufacturers provide both net FVA ratings and ventilation recommendations for their products. In order to estimate, you'll need to know the total square footage of your attic (not including
eaves) and possibly the slope of your roof.